The inclusion of sand with the well fluids produced from an unconsolidated subterranean oil or gas producing zone has long been a problem in the petroleum industry. It can cause erosion of production equipment and can plug the well, causing reduced production levels or loss of well production entirely.
An effective means of combating the problem is the gravel pack, which involves placing a tubular liner in the well bore and packing gravel around it. The liner has slots or other apertures in its walls which are smaller in size than the gravel particles so as to permit the flow of formation fluids while preventing entry of the particles. Typically, the gravel particles are designed to be of a size that will exclude median formation grain size. Thus the gravel pack and screen are designed for absolute exclusion of formation particles and gravel particles from the liner.
In addition, modern methods of predicting gravel size requirements have resulting in reduced gravel pack damage caused by formation particle invasion. Despite improved gravel pack technology, however, damaged gravel packs continue to be a problem. Even with proper gravel sizing, any porosity fluctuation resulting from fluidization with an associated change in the steady rate of fluid flow may cause formation particle release or failure of the formation with its subsequent flow into any open porosity or void in the perforations or annulus. Complete failure of the gravel pack may occur when a pressure surge causes movement of the gravel up the lap area, thereby exposing the liner directly to sand particles, which it is not designed to retain. The lap area in this case is the annular space between the top of the gravel pack and the packer through which the production tubing extends.
The most common cause of severe pressure surges is the shut-in of a well. Normally, when production commences after a gravel pack has been installed the pressure inside the liner will decrease due to the various pressure drops encountered by the fluid as it flows through the well. As a result, fluid will flow down from the lap area through the gravel pack and into the liner to equalize the pressure. When the well is shut in, however, the well bore pressure builds up to essentially the reservoir pressure and, for the pressure of the lap area to equalize to the reservoir pressure, fluid flow will occur from the formation up the gravel pack and into the lap area. If the pressure build-up is too rapid, the fluid velocity up the lap area can be great enough to mobilize or fluidize the gravel. It should be understood that the use of either term "mobilize" or "fluidize" in the specification and claims is not intended to be restricted to any particular type of gravel movement but to pertain to flowable movement in general, including movement as a result of the particles being suspended in a carrier medium.
The significance of the problem can be placed in perspective when it is realized that wells are shut in many times each year, some due to operating requirements but many more due to being instantaneously shut in by emergency shut-down systems, which become operative in response to pressure fluctuations or facility upsets or to adverse weather conditions. When such wells are returned to production, a decrease in productivity may occur, possibly even to the extent of completely losing production, due to sanding-up of the well.
It has been suggested that by determining the minimum velocity required to cause fluidization of a gravel bed, the minimum shut-in time which produces this velocity can be calculated. Then, by taking steps to ensure that the shut-in process is longer in duration than the calculated minimum shut-in time, fluidization can be prevented. As a practical matter the implementation of such a shut-in process is not only time consuming but is not possible in the many emergency instantaneous shut-in situations referred to above.
It has also been suggested that a consolidated gravel pack capable of withstanding severe pressure surges and fluid velocities without fluidizing the gravel be employed to solve this problem. Consolidated gravel packs can be implemented by utilizing gravel which has been coated with uncured resin or by incorporating a liquid resin system in a normal gravel pack slurry. Consolidated gravel packs have the advantage of permitting rapid shut-in, but are significantly more expensive than ordinary gravel packs. Further, consolidation systems have the disadvantage of lower permeability and porosity and possible formation damage due to coating failure when subsequent chemical stimulation is required. Air curing is also necessary in many cases to develop a high strength resin bond which will not fail. Curing the resin systems under in situ conditions can also result in a less competent resin coating.
It would be highly desirable to be able to prevent fluidization or mobilization of normal gravel packs without interfering with the action of emergency shut-down systems which cause instantaneous shut-in of a well. It would also be advantageous to be able to accomplish this in an economical and reliable manner.